The present invention concerns the transmission of speech and is particularly concerned with mobile telephone services. The basic format for a network for mobile speech services will comprise the actual mobile handsets, a base station with which the handsets communicate via a radio link, a mobile switching center (MSC) connected to the base station, and a public network, possibly including the PSTN, to which the MSC is connected. There will, of course, normally be many handsets associated with each base station, and each MSC will be linked to a number of base stations.
There has been an explosive growth in the provision of mobile telephone services in the United Kingdom and this growth is likely to be limited more by the available radio spectrum rather than by saturated demand. This is because telephone terminals are increasingly being connected to their respective networks by radio links. Whether the intention is to provide the user with unrestricted mobility (mobile service), or merely to give the user the freedom of a restricted range of movement (cordless phone), the technical requirements have in common the need to restrict the use of bandwidth on the radio path in order to accommodate large numbers of users.
In an attempt to maximize the use of available bandwidth two emerging standards have been proposed. These standards are known respectively as GSM and DECT. GSM stands for Group Speciale Mobile and DECT stands for Digital European Cordless Telephony and is a standard being developed by ETSI.
GSM achieves its efficiency at the air interface by taking a short sample of speech and analyzing it and then sending data which will allow this burst to be reconstituted. The DECT standard proposed by ETSI has a very similar format. It has currently been proposed that in utilizing either of these two standards a number of encoded speech channels will be multiplexed at the A interface between the base station and the mobile switching center (MSC). The multiplexed signal would then be converted to a 64 kb/s format at the interface between the MSC and the public network to which the MSC is connected.
There are a number of drawbacks to this proposal. Firstly, in order to carry it out, conversion equipment will be required at entry to and exit from the fixed network. This conversion equipment will almost certainly have to be from a common pool. Secondly, there will be a time delay of around 100 ms per round trip due to the requirement for decoding and subsequent encoding. Thirdly, the trunk network loses the bandwidth advantage of the GSM coding.
The present invention is concerned with alleviating the above mentioned disadvantages.
Accordingly, from a first aspect the present invention comprises a speech transmission network having a plurality of nodes where transmission between certain of the nodes is via a predetermined telephony standard. The network comprises a plurality of mobile stations capable of receiving and transmitting over radio links speech which has been analyzed to provide data strings of a predetermined length from which the speech signal can be reconstituted, a base station for communicating with the mobile stations over the radio links and with a mobile switching center, and an interface between each base station and the mobile switching center at which the individual data strings are packetized for transmission across the communication network to another node without conversion to the predetermined telephony standard if the recipient node can reconstitute the speech directly from the data strings.
In accordance with a feature of the invention where the recipient node is not capable of reconstituting the speech from the data strings, conversion to the predetermined telephony standard will be carried out at the most appropriate point in the data network.